In many porous substrates, such as nitrocellulose sheets, cellulose-based papers, and porous polymer sheets, liquids travel laterally along the substrate sheet. The flow is generally capillary. Such sheets are exploited in many applications in the field of diagnostics, such as in biosensors and immunoassay-lateral-flows. In these applications, a strip has been used, in which the liquid travels laterally along the entire width of the strip, cut from a substrate sheet. In multi-analysis-tests, in which the sample liquid must be transported to several reaction/detection areas, it is advantageous for it to be possible to form the substrate sheet in such a way that the sample liquid travels in only specific parts of the sheet, i.e. structural layers guiding the liquid flow are formed in the sheet.
Structural layers guiding the liquid flow can be manufactured in porous substrate sheets using many different methods. Patent US 2009/0298191 A1) (Lateral flow and flow-through bioassay devices based on patterned porous media, methods of making same and methods of using same) discloses, among others, the following methods:                A substrate sheet is saturated with a photoresist, exposed to UV light through a photo-mask defining the liquid channels, and finally developed, when the photoresist is dissolved off the locations of the liquid channels. In this way, areas saturated with photoresist are created, which define the edges of the liquid channels.        A hardening polymer, e.g., polydimethylsiloxane (PDMS), is spread on a stamp, the relief pattern of which defines the boundary areas of the liquid channels. After this, the stamp is pressed onto the substrate sheet, for example, for 20 seconds. Finally, the stamp is removed and the polymer is hardened.        Liquids, which are either hydrophobic themselves, or which can convert the substrate sheet to become hydrophobic, can be applied on the substrate sheet according to a desired pattern, for example, using the following methods: spraying the liquid through a stencil, by silkscreen printing, by inkjet printing, or using a plotter.        The desired areas of the substrate are saturated to become hydrophobic by absorbing wax with the aid of heat.        
In the publication D. A. Bruzewicz, M. Reches, and G. M. Whitesides, ‘Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper’, Anal. Chem., 2008, 80 (9), 3387-3392, barrier lines guiding liquid flow are manufactured using a PDMS solution as an ink in the pen of a plotter.
With the exception of the photoresist-based method, the precision of the edges of the liquid-flow channels are a problem in the aforementioned methods according to the prior art. Because the liquid, which alters the substrate sheet in such a way as to guide a liquid flow, must be absorbed through the entire substrate sheet, it also spreads at the same time laterally and thus the edges of the liquid-flow channel do not become precise.
The publication K. Abe, K. Suzuki, and D. Citterio, ‘Inkjet-printed microfluidic multianalyte chemical sensing paper’, Anal. Chem., 80 (18), 6928-6934, 2008, discloses a method, in which the paper in first saturated with a 1.0 w-% polystyrene-toluene solution, dried, and the liquid channels are finally etched open by inkjet printing with toluene. The inkjet printing generally has to be repeated 10-30 times to achieve a sufficient etching depth, which makes it difficult to use the method in roller-to-roller manufacturing processes.
All of the aforementioned manufacturing methods according to the prior art are quite slow and thus difficult to use in industrial mass-manufacturing processes. In patent US 2009/0298191 A1, it is estimated that patterning a single 10×10 cm substrate sheet using a photoresist-based method takes about 8-10 minutes and with a method using a stamp about 2 minutes.